Add dev-qt from an older version of the gentoo repo

Qt5 has been removed, we restore it since we still use it.

dev-qt/qtconcurrent/qtconcurrent-5.15.18.ebuild

@@ -0,0 +1,23 @@
+# Copyright 1999-2025 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+if [[ ${PV} != *9999* ]]; then
+	QT5_KDEPATCHSET_REV=1
+	KEYWORDS="amd64 arm arm64 ~hppa ~loong ppc ppc64 ~riscv x86"
+fi
+
+QT5_MODULE="qtbase"
+inherit qt5-build
+
+DESCRIPTION="Multi-threading concurrence support library for the Qt5 framework"
+
+IUSE=""
+
+DEPEND="=dev-qt/qtcore-${QT5_PV}*:5="
+RDEPEND="${DEPEND}"
+
+QT5_TARGET_SUBDIRS=(
+	src/concurrent
+)